Gas hydrates are clathrate-type structures consisting of hydrogen-bonded water molecules that form “cages” that are stabilized by captured gas molecules like methane, propane, CO2. Gas hydrates are normally formed under conditions of low temperatures and high pressures. During production of oil and gas, water is often co-produced resulting in a multi-phase system containing oil, water and gas. In addition also solid organic and inorganic particles may be present.
A large and increasing part oil and gas fields are situated subsea at large depths (high pressure) and low temperatures like in the Arctic where the formation of hydrates is very likely to occur, unless prevented by some artificial means. The formation of gas hydrates may lead to increased viscosity and potentially plugging of the well or process facilities including pipelines and valves. Hydrate plugs are a potential hazard which can lead to destruction of equipment and loss of life. In addition, remediation is both time consuming and expensive. Techniques for preventing the formation of gas hydrates or having a controlled hydrate formation are many. They include heating and/or insulation of the pipeline, production outside the hydrate region of the phase diagram, and finally the use of chemical inhibitors. Chemical inhibitors are used to remove hydrates, prevent hydrates from forming, or prevent hydrate particles to agglomerate and plug process equipment.
Often a combination of heating/insulation, depressurization etc. with chemical gas hydrate inhibitors are used. Hydrate inhibitors are divided into two main groups, Thermodynamic inhibitors and Low dosage inhibitors. The Thermodynamic hydrate inhibitors, THIs, are chemicals which shift the thermodynamic equilibrium of hydrates towards lower temperatures. They are added at very high concentrations, 10 to 60% relative to water, and facilities for their regeneration and recycling is therefore often required. The most common thermodynamic inhibitors used today are methanol and monoethylene glycol. Diethylene glycol is also used, but is less powerful. Triethylene glycol and ethanol are mainly used for removing smaller amounts of water from flow lines and process facilities so that hydrates cannot form.
The Low dosage inhibitors (injection rates less than 5%) are divided into kinetic inhibitors (KHI) and anti-agglomerates (AA). The KHIs are a class of molecules which delays the formation of gas hydrates for a period of time which is dependent on different process parameters like the sub-cooling, pressure and fluid composition. The polymers which constitute kinetic hydrate inhibitors do most often contain amide groups which make them polar, and a hydrocarbon chain which is adjacent to or directly bonded to the amide.
AAs do not prevent the formation of hydrate particles but stop them from agglomerating. Instead of plugging the hydrate particles are stabilized in and transported dispersed in the oil phase. Gas hydrate prevention by AAs therefore requires a continuous oil phase. AAs are surfactants of various molecular weights, like quaternary ammonium salts. Quaternary ammonium salts are generally very toxic and only partially biodegradable.
Effective hydrate inhibitors which are non-toxic and biodegradable, so called green” alternatives, are at present not commercially available. A few examples of biomolecules, like anti-freeze proteins and bio surfactants, which may function as gas hydrate inhibitors have been described in the literature.
This present invention relates to a new type of LDHIs which are both non-toxic and biodegradable. The molecular structure is of the type hyperbranched polyester polyol. Hyperbranched polymers are a class of polymers which extend radially in space with one potential branch point per repeating unit instead of forming long chains, like traditional linear polymers. The high degree of branching renders the structure compact and gives rise to a multitude of functional end groups which are available to chemical modification. Hyperbranched polymers have previously been used as gas hydrate inhibitors and some examples are given below.
One class of commercial KHIs is the hyperbranched poly(ester amide)s; cf. Villano et al, described in Chemical engineering Science 64 (2009) 3197-3200. The patent, WO/01/77270 describes adding to the mixture an amount of a dendrimeric compound effective to inhibit formation of hydrates at conduit temperatures and pressures, and flowing the mixture containing the dendrimeric compound and any hydrates through the conduit. Preferably, a hyperbranched polyester amide is used as hydrate formation inhibitor compound.
WO/2008/017007 discloses use of low dose gas hydrate inhibitors for controlling the gas hydrate formation in a well completion fluid in the annular space of hydrocarbon producing well. The low dosage gas hydrate inhibitors listed include low dosage hydrate inhibitors (LDHIs), kinetic hydrate inhibitors, dendrimeric or branched compound, linear polymers and copolymers and onium compounds. A particularly useful group of hydrate inhibitors include dendrimeric compounds and in particular hyperbranched polyester amides are mentioned. The highly branched dendrimeric compounds may have a number average molecular weight in the range of from about 100 to about 5000, with a molecular weight distribution of as broad as 2 to about 30.
US2006/0106265 describes a method for inhibiting formation of hydrocarbon gas hydrates. In this method a composition including at least one dendrimeric compound having a number average molecular weight of at least 1000 atomic mass unit is added. Preferably the dendrimeric compound is a branched or cross linked polymer. Hyperbranched polyester amides are mentioned as suitable compounds.
WO01/77270 relates to a method for inhibiting the plugging of a conduit, by inhibiting formation of hydrates. A dendrimeric compound effective to inhibit formation of hydrates is used. Preferably a hyperbranched polyesteramid where the condensation polymer contains ester groups and at least one amid group in the backbone and has a number average molecular between 500 and 50000 is used. The functional end groups (hydroxyl groups) of the polycondensation product can be modified by further reactions. Suitable modification can take place by reaction of at least part of the hydroxyl end groups with fatty acids.
US 2010/0018712 describes apparatus and methods for inhibiting formation of hydrocarbon hydrates and/or agglomerates. The low dosage hydrate inhibitors may be a kinetic inhibitor selected from the group consisting of dendrimeric compounds, hyperbranched polymers, linear polymers and copolymers.
US 2006/0218852 describes compositions useful for controlling formation of hydrates in various fluid systems. The composition comprises polymeric materials that are dendritic in nature or hyper-branched polyamino polymers.
Many of the hydrate inhibitors described in the prior art are based on polymers that are not environmentally friendly meaning either being toxic or non-degradable or both.
U.S. Pat. No. 5,418,301 relates to a dendritic macromolecule of the polyester type comprising a central initiator molecule or initiator polymer having one or more reactive hydroxyl groups. The dendritic macromolecule is intended for use as a component in applications such as alkyds, alkyd emulsions, saturated polyesters, unsaturated polyesters, epoxy resins, phenolic resins, polyurethane resins, polyurethane foams and elastomers, binders for radiation curing systems such as systems cured with ultra-violet (UV) and infra-red (IR) light or electron-beam (EB), dental materials, adhesives, synthetic lubricants, microlithographic paints, binders for powder systems, amino resins, composites reinforced with glass, aramid or carbon/graphite fibres and moulding compounds based on urea-formaldehyde resins, melamineformaldehyde resins or phenol-formaldehyde resins.
The inventors of the present invention have found that by modifying biodegradable and non-toxic hyperbranched polyester polyols by replacing one or more of the hydroxyl groups to obtain an amphiphilic structure, these modified polymers are useful for use as gas hydrate inhibitors.